[boomanaiden154]

It's not called AutoFDO. AutoFDO refers to a specific sampling-based profile technique out of Google (https://dl.acm.org/doi/abs/10.1145/2854038.2854044). Sometimes people will refer to that as PGO though (with PGO and FDO being somewhat synonymous, but PGO seeming to be the preferred term in the open source LLVM world). Chrome specifically uses instrumented PGO which is very much not AutoFDO.

PGO works just fine in Rust and has support built into the compiler (https://doc.rust-lang.org/rustc/profile-guided-optimization....).

[vlovich123]

I wasn’t trying to conflate the two. PGO traditionally meant a trace build but as a term it’s pretty generic, at least to me to the general concept of “you have profile information that replaces generically tuned heuristics that the compiler uses). AutoFDO I’d classify as an extension to that concept to a more general PGO technique; kind of ThinLTO vs LTO. Specifically, it generates the “same” information to supplant compiler heuristics, but is more flexible in that the sample can be fed back into “arbitrary” versions of the code using normal sampling techniques instead of an instrumented trace. The reason sampling is better is that it more easily fits into capturing data from production which is much harder to accomplish for the tracing variant (due to perf overheads). Additionally, because it works across versions the amortized compile cost drops from 2x to 1x because you only need to reseed your profile data periodically.

I was under the impression they had switched to AutoFDO across the board but maybe that’s just for their cloud stuff and Chrome continues to run a representative workload since that path is more mature. I would guess that if it’s not being used already, they’re exploring how to make Chrome run AutoFDO for the same reason everyone started using ThinLTO - it brought most of the advantages while fixing the disadvantages that hampered adoption.

And yes, while PGO is available natively, AutoFDO isn’t quite as smooth.

[boomanaiden154]

I'm not sure where you're getting your information from.

Chrome (and many other performance-critical workloads) is using instrumented PGO because it gives better performance gains, not because it's a more mature path. AutoFDO is only used in situations where collecting data with an instrumented build is difficult.